Recent genome-wide association scans have resulted in the identification of a large number of genes which are associated with autoimmunity. Identifying the functional outcome of these genetic variants is an important step in understanding the underlying mechanisms of these diseases. Several genetic variants have been linked to multiple diseases, indicating that common mechanisms are at play in these diseases. One such genetic variant is the PTPN22 1858T. This variant is associated with T1D, RA, Graves Disease, myasthenia gravis and SLE. This variant leads to a single amino acid change in the phosphatase Lyp R620W, resulting in a dominant gain of function. We and others have demonstrated that this variant results in a blunted response to activation via the T cell receptor which suggests a mechanism by which this variant may enhance an individual's predisposition to autoimmunity. In this grant we propose to address the hypothesis that the impaired T cell receptor signaling resulting from the LypR620W variant shapes CD4 T cell function in a manner which favors the development of proinflammatory memory responses through the production of proinflammatory cytokines, a resistance to AICD and an expansion of the CD4 memory T cell population in the peripheral blood. We have focused these studies on those which can be performed with human subjects as the molecular interaction altered by this single amino acid change may be unique to human cells. We will address the impact that the Lyp R620W variant has on the differentiation, lineage commitment, proliferation and survival of human CD4 T cells in vitro and establish the underlying cellular and molecular mechanisms by which this variant alters the immune response. We will then address the global impact of this variant on the immune response in vivo by comparing the response to vaccination with pneumococcal vaccine conjugated to the carrier protein CRM197 in individuals who carry the disease associated variant to those of similarly matched subjects who do not. This will be done first with healthy subjects, followed by studies of individuals with T1D. We propose to do this in three specific aims: Aim 1) We will address the functional impact of the PTPN22 1858T variant in isolation, by performing experiments on T cells derived from healthy controls who carry the risk variant in vitro. Aim 2) We will test the hypothesis that individuals who possess the PTPN22 1858T allele develop an enhanced proinflammatory memory T cell response upon antigen challenge in vivo. Aim 3) We will examine how phenotypes associated with the LypR620W variant are expressed in the context of autoimmune diabetes in vitro and in vivo, then we will extend these studies to the impact of PTPN22 1858T variant on the islet specific T cell response in these subjects.